The purpose of the proposed investigation is to continue our study into the ways in which lanthanide ions can be used to probe protein structure. The varied spectral and magnetic properties of these ions can be used to determine binding sites and distances between inhibitors and the metal ions. They can also be used to detect conformational changes in the protein due to a variety of perturbants introduced into solution via the alterations in the measured distances. A determination of the stereochemistry of the substrate or inhibitor bound to the active site will also be attempted by measurement of distances from different parts of the inhibitors chosen to one or more metal ions. By utilizing two well-known proteins, thermolysin and trypsin, a continuing effort will be made to determine under what conditions the equations governing nmr relaxation measurements and fluorescence energy transfer measurements are valid for calculating distances in protein systems. The similarity in size and chemical behavior of the calcium and lanthanide ions should allow the results obtained in the protein-lanthanide system to be extended to the protein-calcium systems, which plays an active role in the chemistry of the body, e.g., blood clotting, muscle contraction, etc. A direct investigation of these systems using the calcium ion is severely hampered by the absence of any readily measurable spectral properties of the calcium ion.